Method and device for measuring oxygen partial pressure

ABSTRACT

The partial pressure of oxygen in a gas mixture is determined by measuring the torque acting on two balloons exposed to a nonuniform magnetic field. One of the balloons is filled with a gas comprising a known proportion of oxygen while the other balloon contains the gas mixture.

O Unlted States Patent 1 1 1111 3,881,152

Tasaki Apr. 29, 1975 [5 METHOD AND DEVICE FOR MEASURING 2.744.234 5/1956Munda'y ct al. 324/36 OXYGEN PARTIAL PRESSURE 3.026.472 3/1962 Greene etal. 324/36 3.612.990 10/197] Dcl Duca 324/36 [76] Inventor: Aku-a Taskl. 1-3-9 3.612.991 10 1971 Greene 324/36 Yamada-higashi. Suita-shi.Osaka-m" Japan Primary E.\'uminerStanley T. Krawczewicz [22] Filed:Sept. 19, 1973 Attorney. Agent, or Firm-Gardiner. Sixbey. Bradford andCarlson [2]] Appl. No.: 398,859

[57] ABSTRACT [52] U.S. Cl 324/36; 73/27 A 51 lm. Cl. G0lr 33/12 ThePuma Pressure of Oxygen a gas [58] Field of Search 324/36; 73/27 Atermined by measuring the torque acting loons exposed to a non-uniformmagnetic field. One of [56] References cued the balloons is filled witha gas comprising a known UNITED STATES PATENTS proportion of oxygenwhile the other balloon contains the gas mixture. 2.416.344 2/1947Pauling 324/36 2.666.893 1/1954 Munday 324/36 4 Claims, 2 DrawingFigures 4 4 2 Q 4 11 \g 2 Q 4 I METHOD AND DEVICE FOR MEASURTNG OXYGENPARTIAL PRESSURE The present invention relates to the measurement of thepartial pressure of oxygen in a gas mixture and, more particularly, to amethod and device for continuously monitoring the partial pressure ofoxygen in a gas mixture by magnetometric means.

The need for a simple and sensitive device for measuring and monitoringthe oxygen partial pressure of air arises from the situation that inmodern, enclosed, airconditioned buildings, the oxygen content of theair in the building can often fall to a dangerously low level because ofan insufficient intake of external air. Similar situations arise insubterranean mining, submarines and scientific submersible devices, highaltitude aircraft and manned space vehicles. It is known that the humanbrain function is impaired when the oxygen content in the breathing airfalls to a level of 15 to 17 percent from the normal level of 21percent.

The underlying principle of many of the known methods and devices formeasuring the oxygen partial pressure of air and other gas mixturescontaining oxygen is based on the fact that oxygen is paramagnetic whilepractically all other gases are weakly diamagnetic. It is notsurprising, therefore, that the oxygen partial pressure of a gas mixturehas been determined by measuring the magnetic susceptibility of the gasmixture. In most instances, however, such measurements have been verytedious requiring expensive instruments.

It is, therefore, the object of the present invention to provide amethod and means for continuously and inexpensively measuring andmonitoring the oxygen partial pressure of gas mixtures such as air.

The magnetic susceptibility of pure oxygen at a pressure of oneatmosphere is (0.1434 i 0.0004) X 10 electromagnetic units per cubiccentimeter at 20 C. This value changes inversely with the absolutetemperature. The force which a non-uniform magnetic field exerts on aparamagnetic substance is given by the formula:

where X represents the magnetic susceptibility per gram of the material,m is the mass of the material, and H is the magnetic field strength. Theforce, F, in this equation is expressed in dynes. The gradient of themagnetic field produced by a simple permanent magnet is of the order of10 oersted 2/cm. Therefore, the force which acts on the volume of oxygenof IO cubic centimeters (14.3 mg) is, according to the above formula, ofthe order of 0.1 dyne. This force corresponds approximately to thegravitational force acting on a mass of 0.1 mg. Since the torque ortwisting moment measurable with a torsion balance under laboratoryconditions is of the order of dyne/cm, the feasibility of utilizingthese factors to construct a simple, reasonably accurate device tomeasure the partial pressure of oxygen in gas mixtures was studied.

Accordingly, the objectives of this invention have been achieved bymeasuring the torque acting on two balloons exposed to a non-uniformmagnetic field, one of the balloons being filled with anoxygen-containing gas as reference and the other of the balloons withthe gas mixture.

The device according to the present invention will be hereinafterdescribed with reference to the accompanying drawings which illustrate,diagrammatically and by way of example only, a preferred embodiment ofthe present invention. In the drawings:

FlG. 1 shows a side elevational view of a device of this invention; and

FIG. 2 is a section on line 22 of FIG. 1 and shows the arrangement ofthe motive portions 10 of the device.

ln FIGS. 1 and 2, the motive portions of the invention, 10, aresuspended from a support member 1] by means of a crossbar l2 and anadjustment bolt 13 fastened thereto. A tubular shaft 14 is connected tobolt 13 by means of torsion assembly 15. Attached to shaft 14 are arms16 which horizontally support two balloons or spheres l7 and 19. Theballoons are rigidly fixed to shaft 14 by means of arms 16 so that anaxis passing through the centers of the balloons is in a plane that isperpendicular to the axis of rotation of torsion assembly l5.

Balloon 17 is a sealed reference or comparison balloon. In operation inan air conditioning system, balloon 17 would contain fresh aircomprising 21 percent oxygen at a pressure of one atmosphere. In themeasuring or monitoring of any other enclosed system comprising oxygenor some other paramagnetic gas, balloon 17 would be filled with thechosen reference gas at conditions of temperature and pressure calledfor by the particular system.

The other balloon, 19, is the analysis balloon and is filled with a gasthe oxygen partial pressure of which is sought to be determined. Thedevice shown in the drawings is designed to be used with an airconditioning system and, therefore, balloon 19 is perforated with anumber of holes 18 so that its interior is constantly exposed to theambient air of the particular enclosed system in which it is placed.

The two balloons are exposed to a non-uniform magnetic field bypositioning the balloons close to the surface of permanent magnet 20.The surface of magnet 20 which faces the balloons may be speciallyconfigured, such as with a longitudinal groove or channel (as shown inFIG. 1), to allow optimum exposure to the magnetic field.

With reference again to the operation of the device shown in thedrawings, as long as the oxygen content of the ambient air remains atthe predetermined level of the reference gas, the magnetic forces actingon balloons l7 and 19 are balanced and the balloons will not be causedto rotate. However, when the oxygen partial pressure of the closedenvironment decreases to a level lower than that of the gas in referenceballoon 17, balloon 19 is caused to move away from magnet 20 as a resultof balloon 17 being attracted more strongly by the magnetic field. Therespective to and fro movement of the balloons results in a smallrotation of shaft 14. The actual rotation takes place in assembly 15which comprises a rigid bar 21 and a fine ribbon or filament 22 which iscapable of being twisted along its longitudinal axis. Bar 21 is firmlymounted to bolt 13 and shaft 14 depends from the lowermost end offilament 22. Filaments similar to that utilized in the practice of thisin vention are used in the manufacture of torsion balances, torsiongalvanometers, torsion electrometers and like scientific and technicalinstruments.

A casing 23 preferably encloses the device of this invention to reducethe chance of error or malfunction caused by even moderate airdisturbance in the vicinity of balloons l7 and 19 and torsion assembly15. The casing is especially required when the device is used incombination with an air conditioning system. However, to provide anaccurate and continuous monitoring of the air quality of such a system,the casing must be provided with vents (not shown) to allow an influxand outflow of the ambient air. These vents may, in turn, be providedwith baffles or other devices to restrict or eliminate air currentswithin casing 23 and to effect an even sampling of the ambient air.

The rotation of shaft 14 may be measured directly or by remote means. Apreferred technique for measuring the rotation is illustrated in FIG. 1and involves the use of a light beam emanating from a source remote fromcasing 23. The light beam passes through window 24 mounted in casing 23and is reflected to photoresponsive device 25 which reads the rotationaldeviation from a zero point. Device 25 may also automatically actuate avalve, pump, or such other apparatus (not shown) which allows admissionof more fresh air or stored oxygen to the closed environment. Reflectionof the light beam in the device may be effected by mirror 26 which isdirectly mounted to shaft 14 so that it rotates simultaneously with itand balloons l7 and 19.

The oscillatory movement of shaft 14 is preferably damped, such as byplate 27 attached to the end of shaft 14 and immersed in a container ofliquid paraffin 28 appropriately positioned on base plate 29. Any othersuitable damping means may be substituted for that shown in thedrawings.

The zero point of the device can easily be determined since the forcesacting on the balloons approach zero as the magnet is moved away fromthe balloons. Thus, magnet is fixed to platform support 30 which isslidably mounted on base plate 29 so that the magnet may be moved towardand away from the balloons. Platform support 30 may be slidablyactuated, for instance, by means of a screw actuator 31 mounted injournal 32. Screw actuator 31 is rotated by means of a worm gear 33mounted on screw shaft 34 which projects downwardly through base plate29 and is provided with knob 35 for manual operation.

It is to be noted that the sensitivity of the device of this inventioncan be varied in a wide range by slightly changing the position ofmagnet 20. By moving the permanent magnet, it is also possible tocompensate for changes in the magnetic susceptibility of oxygenassociated with temperature changes. However, the effect of temperatureon the magnetic susceptibility is negligible, being approximately 3percent for a change of 10 C.

A device according to the present invention has been constructed andinstalled for testing with the air conditioning unit of a modern,enclosed building. The balloons were made of celluloid and had diametersof 35 mm. Both balloons weighed 2.4 grams after one of them waspunctured with about ten holes and the other was filled with fresh airat about one atmosphere. These balloons were affixed to a cupro-nickeltubular shaft at a center-to-center distance of mm. The torsion assemblycomprised a copper bar and a phosphorus bronze filament measuring 12 mmlong, 0.141 mm wide and 0.015 mm thick. A permanent magnet having theoverall dimensions of 12 cm X 7 cm X 5 cm was used. The damping ofoscillation was satisfied by using a 18 mm X 12 mm phosphorus bronzeplate at the end of the cupro-nickel shaft. During the testing of theabove device, the oxygen content of the air in the building wasdetermined with an accuracy of better than 1 percent. It is to beexpected, however, that the sensitivity of this device can be increasedby a factor up to about 5 by reducing the weight of the balloons and byusing a permanent magnet of special design and configuration.

What is claimed is:-

l. A method for measuring the partial pressure of oxygen in an ambientgas mixture comprising the steps of exposing two balloons to anon-uniform magnetic field, one of said balloons enclosing anoxygen-containing gas as reference, the other of said balloons having aseries of perforations to provide a balloon filled with said ambient gasmixture, and said magnetic field acting upon said balloons from the samedirection, and measuring the torque acting on said two balloons.

2. A method according to claim 1, wherein one of said balloons containsair comprising 21% oxygen and the other of said balloons is filled withthe ambient air of an enclosed, air conditioning system.

3. A device for measuring the partial pressure of oxygen in an ambientgas mixture comprising a first balloon enclosing an oxygen-containingreference gas, a second balloon having a series of perforations, saidperforations providing means for filling said balloon with said ambientgas mixture, torsion means to which said balloons are horizontallymounted, said mounted bal loons being rotatable in a plane perpendicularto the axis of torsion, means for establishing a non-uniform magneticfield through said balloons so that said balloons are acted upon byforces in the same direction, and means co-oper ating with said torsionmeans for measuring the torque acting on said balloons by reason of saidnon-uniform magnetic field.

4. A device according to claim 3, wherein said first balloon is filledwith air comprising 21 percent oxygen and said second balloon is filledwith ambient air of an enclosed, air conditioning system.

1. A method for measuring the partial pressure of oxygen in an ambientgas mixture comprising the steps of exposing two balloons to anon-uniform magnetic field, one of said balloons enclosing anoxygen-containing gas as reference, the other of said balloons having aseries of perforations to provide a balloon filled with said ambient gasmixture, and said magnetic field acting upon said balloons frOm the samedirection, and measuring the torque acting on said two balloons.
 2. Amethod according to claim 1, wherein one of said balloons contains aircomprising 21% oxygen and the other of said balloons is filled with theambient air of an enclosed, air conditioning system.
 3. A device formeasuring the partial pressure of oxygen in an ambient gas mixturecomprising a first balloon enclosing an oxygen-containing reference gas,a second balloon having a series of perforations, said perforationsproviding means for filling said balloon with said ambient gas mixture,torsion means to which said balloons are horizontally mounted, saidmounted balloons being rotatable in a plane perpendicular to the axis oftorsion, means for establishing a non-uniform magnetic field throughsaid balloons so that said balloons are acted upon by forces in the samedirection, and means co-operating with said torsion means for measuringthe torque acting on said balloons by reason of said non-uniformmagnetic field.
 4. A device according to claim 3, wherein said firstballoon is filled with air comprising 21 percent oxygen and said secondballoon is filled with ambient air of an enclosed, air conditioningsystem.